VMware ESXi and vSphere Cluster Management

Virtual Machine Definition and Virtual Hardware in VMware ESXi

Learn what a virtual machine is, how ESXi manages it, which virtual hardware it uses, and how .vmx and .vmdk files define and store a VM.

A virtual machine (VM) is a software-defined computer. It provides virtual processors, memory, storage, networking, firmware, and other devices so that it can run an operating system and applications much like a physical computer.

The operating system inside the VM is called the guest operating system. The physical server that provides the underlying resources is the host. These are different systems: the guest runs inside the VM, while the host runs VMware ESXi and supplies the physical CPU, memory, storage, and network resources.

This lesson explains the relationship between a VM, its virtual hardware, the ESXi hypervisor, and the files stored on an ESXi datastore.

What Is a Virtual Machine?

A VM is an isolated software environment that behaves like a computer. An administrator assigns it virtual CPUs, memory, disks, network adapters, and other devices. The guest operating system detects those devices and uses them through its normal hardware drivers.

For example, a Linux application server VM may contain a Linux guest operating system, a web server, application software, virtual disks, and a virtual network adapter. Linux does not normally need to know which physical server model is running ESXi. It works with the virtual hardware presented to it.

A physical server can run multiple VMs at the same time. Each VM has its own guest operating system, applications, configuration, and virtual disks. ESXi keeps the VMs logically separate while sharing the host's physical resources.

Host, Hypervisor, VM, and Guest Relationship

A hypervisor is the software layer that manages virtual machines and separates them from physical hardware. VMware ESXi is a bare-metal hypervisor: it is installed directly on a physical server rather than running as an application inside another operating system.

The physical server running ESXi is the ESXi host. ESXi schedules CPU work, assigns and manages memory, provides access to datastores, and connects virtual network adapters to virtual and physical networking.

The guest is the operating system and applications running inside a VM. The VM itself includes the virtual hardware configuration and the files that represent its storage and configuration.

The relationship can be summarized as follows:

  1. Physical host hardware: physical processors, RAM, storage interfaces, disks, and network adapters.
  2. ESXi hypervisor: manages access to the physical hardware and provides hardware abstraction.
  3. VM virtual hardware: virtual CPUs, memory, disks, controllers, NICs, firmware, and display devices.
  4. Guest operating system: uses the virtual hardware as its computer platform.
  5. Applications: run inside the guest operating system.

Physical Computer and Virtual Machine Comparison

Physical computer component — Virtual machine equivalent — Purpose

Physical processor — Virtual CPU: provides processing capacity to the guest operating system. ESXi schedules the virtual CPU on physical processor resources.

Physical RAM — Virtual memory: provides working memory to the guest. The configured amount is a VM resource allocation, while ESXi manages its use on the host.

Physical disk — Virtual disk: provides persistent storage to the guest. The virtual disk is represented by files or related storage objects on a datastore.

Physical NIC — Virtual NIC: provides network connectivity to the guest through ESXi virtual networking and, ultimately, physical network adapters.

Storage controller — Virtual SCSI or IDE controller: gives the guest a device through which it accesses virtual disks and optical drives.

Physical firmware — Virtual firmware such as Phoenix BIOS: starts the VM and provides firmware services during boot.

Virtual Hardware Components

Virtual hardware is the collection of software-presented devices assigned to a VM. The configuration is created per VM, and each device consumes, represents, or maps to host resources.

Virtual CPU

A virtual CPU, often called a vCPU, is a processing unit presented to the guest. If a VM is configured with two vCPUs, the guest sees two processors or processor cores according to the guest's hardware view. ESXi schedules their work on physical CPU resources.

The CPU vendor presentation may appear as AMD or Intel depending on the physical host hardware and the compatibility context. This presentation can matter to operating systems and software that inspect processor features.

Virtual Memory

Virtual memory assigned to a VM is the amount of RAM made available to its guest operating system. The guest uses it as ordinary computer memory. ESXi manages the relationship between this allocation and the physical host's available memory.

Virtual Disk

A virtual disk is storage presented to the guest as a disk device. The guest can partition it, format it, and place an operating system or application data on it. Its data is stored through one or more related virtual disk files or storage objects.

Virtual Storage Controller

A virtual storage controller connects the guest's virtual disks to the VM. Common legacy controller choices include BusLogic Parallel, LSI Logic Parallel, and LSI Logic SAS. A guest operating system needs a compatible driver to use the selected controller.

Virtual Network Adapter

A virtual NIC is the VM's network interface card. ESXi connects it to virtual networking, such as a virtual switch and port group, and then to physical network connectivity when appropriate. Legacy emulated choices include Intel E1000, Intel E1000e, and AMD PCnet.

CD-ROM Device

A virtual CD-ROM drive can connect to an ISO image, a datastore-based image, or a physical optical device when supported. It is commonly used to boot an installer or provide removable media to the guest.

Firmware and Motherboard

The VM has virtual firmware and a virtual motherboard model. The firmware starts the VM and supports the boot process. A legacy firmware option presented to certain VMs is Phoenix BIOS. The virtual motherboard is generally described as an Intel-compatible motherboard.

Video Adapter

The standard virtual display device is a Standard VGA video adapter. It provides basic display output to the guest and is sufficient for many server administration and installation tasks.

How ESXi Provides Hardware Abstraction

Hardware abstraction means that the VM interacts with standardized virtual devices instead of directly depending on the exact physical devices in one server. The guest sees a virtual CPU, virtual disk controller, virtual disk, and virtual NIC. ESXi translates and schedules requests for the physical host.

For example, when a guest writes data to a virtual disk, ESXi handles the request and sends it to the appropriate datastore storage. When a guest sends network traffic through its virtual NIC, ESXi passes that traffic through its virtual networking configuration and physical network path.

ESXi can allocate physical resources according to the VM configuration and current workload. A VM's configured vCPU count and memory size establish its virtual hardware, while ESXi determines when and where the corresponding work uses host resources.

Virtual Hardware Presented to an ESXi VM

Firmware — Phoenix BIOS: legacy virtual firmware option; the available firmware depends on the VM configuration and compatibility level.

Motherboard — Intel-compatible motherboard: provides the virtual platform presented to the guest.

IDE controller — Intel PCI IDE controller: legacy storage controller used by compatible virtual devices.

CD-ROM drive — IDE CD-ROM drive: presents optical media or an ISO image to the guest.

SCSI controller — BusLogic Parallel, LSI Logic Parallel, or LSI Logic SAS: connects virtual disks and requires guest driver support.

CPU — AMD or Intel presentation: depends on physical host hardware and compatibility context.

Network adapter — Intel E1000, Intel E1000e, or AMD PCnet: emulated network models with different guest compatibility characteristics.

Video adapter — Standard VGA: basic virtual graphics device for guest display output.

The visible devices are not identical for every VM. They can vary with the VM hardware version, supported guest operating system, firmware choice, and administrator-selected configuration.

Virtual SCSI and Network Adapter Options

SCSI controllers — BusLogic Parallel, LSI Logic Parallel, LSI Logic SAS: choose a model supported by the guest operating system and its installer. Controller selection affects whether the guest can detect its virtual disks.

Network adapters — Intel E1000, Intel E1000e, AMD PCnet: choose a model for which the guest has a suitable driver. Legacy emulation can improve compatibility but is not automatically the best choice for every workload.

Why Virtual Hardware Selection Matters

The guest operating system needs a compatible driver for each virtual device. A device can exist in the VM configuration but still be unusable if the guest cannot identify it or lacks its driver.

Storage-controller selection is especially important during operating system installation. If the installer does not include a driver for the selected virtual SCSI controller, it may report that no disk is available even though a virtual disk is attached.

Network-adapter selection also affects deployment. A guest without a driver for its configured NIC may boot successfully but have no network connectivity. Before changing an adapter model, verify guest support and plan for any required driver or network reconfiguration.

Legacy emulated devices such as IDE, BusLogic, E1000, and PCnet can be useful for older operating systems. Newer or specialized configurations may provide different options, but compatibility should always be checked before changing a boot controller or network adapter.

VM Files on an ESXi Datastore

A datastore is a storage location accessible to ESXi. A VM is normally represented by multiple related files stored in a VM directory on a datastore. The configuration and the disk data are separate concepts, even though they work together as one VM.

The .vmx Configuration File

The .vmx file is the primary VM configuration file. It records settings such as virtual hardware definitions, device connections, firmware-related settings, and references to virtual disks.

The .vmdk Virtual Disk File

A .vmdk file represents or describes a VM virtual disk. Depending on the disk format and storage arrangement, a virtual disk can involve a descriptor and separate data files. Therefore, the visible disk representation may consist of multiple related files rather than one standalone file.

Core ESXi Virtual Machine Files

.vmx configuration file — defines the VM's virtual hardware and configuration. Keep it consistent with the disk references and other VM metadata.

.vmdk virtual disk file — stores or describes the contents of a virtual disk. Preserve all associated disk files and do not rename or copy only one component without understanding its dependencies.

When copying, restoring, or migrating a VM, treat its files as a managed unit. A VM may fail to start if its .vmx file references a missing disk, if a related .vmdk file is renamed, or if files are incomplete or inaccessible.

Practical Example: A Linux Application Server VM

Suppose an administrator creates a Linux application server VM with two virtual CPUs, 8 GB of virtual memory, one or more virtual disks, a virtual SCSI controller, and a virtual NIC.

  • The Linux guest sees processors, memory, a disk controller, disks, and a network adapter as its computer hardware.
  • ESXi schedules the VM's CPU work on physical processors and maps its memory use to host RAM.
  • ESXi provides access to the virtual disks through the datastore and connects the virtual NIC through virtual networking.
  • The VM's hardware and device references are represented in a .vmx file.
  • The virtual disk contents are stored through associated .vmdk files or related datastore objects.

VM Isolation and Portability

Because a VM uses virtual hardware, it has a degree of independence from the exact physical server on which it runs. The guest can continue to use the same virtual CPU, disk, controller, and NIC model when the VM's execution location changes.

VM migration is the relocation of a VM's compute placement, storage placement, or both within compatible virtual infrastructure. A VM may be moved between physical hosts, and its virtual disks may be moved between storage locations or supported storage types.

Portability does not mean that every VM can run on every server without preparation. A successful move depends on compatible hosts, suitable CPU and VM hardware compatibility, destination datastore access, network availability, supported migration features, and the VM's current state and configuration.

Moving a VM to Another ESXi Host

When a VM moves to another ESXi host, it can continue using the same configured virtual hardware model even though different physical processors, memory modules, storage paths, and network adapters perform the work. This demonstrates the separation between the guest and a particular physical server.

Actual migration requires compatible hosts and an appropriate supported method. The destination must have access to the VM's storage, or the migration method must provide storage migration. The required virtual networks and other infrastructure resources must also be available.

Moving Virtual Disks

Virtual disks can be relocated between supported storage locations or storage types. The VM configuration must continue to reference the correct disk location, and all related disk components must remain available. Storage migration capabilities and compatibility requirements determine which moves are supported.

Troubleshooting Virtual Hardware and VM Files

The Guest Installer Cannot See the Virtual Disk

  • Verify that the selected virtual SCSI controller is supported by the guest operating system.
  • Check whether the installer contains the required storage-controller driver.
  • Confirm that the virtual disk is attached, connected, and assigned to the expected controller and device node.
  • Do not change a boot disk controller without checking the guest's driver support and recovery plan.

The Guest Has No Network Connectivity

  • Check whether the guest has a driver for the configured Intel E1000, Intel E1000e, AMD PCnet, or other presented adapter.
  • Confirm that the virtual NIC is connected and attached to the correct virtual network or port group.
  • Check the guest's IP configuration, routes, and other network settings.

The VM Does Not Start After Files Were Copied or Altered

  • Confirm that the .vmx file references the correct virtual disk paths.
  • Check that all associated .vmdk files are present, complete, accessible, and correctly named.
  • Remember that a virtual disk may use multiple related files.
  • Use supported backup, restore, migration, and datastore-management workflows when possible instead of independently copying individual files.

A Move to Another Host or Datastore Fails

  • Check host compatibility and the VM's hardware compatibility requirements.
  • Confirm that the destination has the required datastore and virtual network access.
  • Verify that the selected migration method supports the VM's current power state and configuration.

Conceptual VM Configuration

Creating a VM involves assigning virtual CPU count and memory size, attaching one or more virtual disks through a selected controller, selecting a compatible virtual NIC, and storing the VM files on an ESXi-supported datastore.

VM configuration
  virtual CPUs: assigned count
  virtual memory: assigned size
  storage controller: selected virtual SCSI or IDE model
  virtual disks: attached through the controller
  network adapter: selected virtual NIC model
  storage location: ESXi-supported datastore

This is a conceptual configuration model rather than a command-line procedure. The exact available choices depend on ESXi, the VM hardware version, the guest operating system, and the infrastructure configuration.

Exam-Relevant Notes

  • ESXi is a bare-metal hypervisor installed directly on a physical server.
  • The host is the physical server running ESXi; the guest operating system runs inside a VM.
  • Virtual hardware is presented to the guest as if it were physical hardware, while ESXi manages the underlying physical resources.
  • The .vmx file primarily defines VM configuration and virtual hardware.
  • The .vmdk file represents or describes virtual disk data and may be associated with additional disk files.
  • BusLogic Parallel, LSI Logic Parallel, and LSI Logic SAS are virtual SCSI controller choices.
  • Intel E1000, Intel E1000e, and AMD PCnet are legacy emulated virtual NIC choices.
  • Phoenix BIOS, an Intel-compatible motherboard, an Intel PCI IDE controller, an IDE CD-ROM drive, and Standard VGA are examples of legacy virtual devices.
  • Portability is enabled by hardware abstraction, but migration still requires compatible hosts, storage, networking, and supported migration methods.

Summary

A VM is a software-based computer that runs its own guest operating system and applications. ESXi sits between the VMs and the physical host, scheduling CPU work, managing memory, providing storage access, and connecting virtual networking.

The guest uses virtual CPUs, memory, disks, controllers, NICs, firmware, a motherboard, a CD-ROM device, and a video adapter. These devices provide hardware abstraction and help make VMs portable, while driver and infrastructure compatibility remain essential.

On an ESXi datastore, the VM configuration is primarily represented by a .vmx file, while virtual disk data is represented through one or more related .vmdk files. For safe copying, restoration, and migration, manage the complete set of VM files as one consistent unit.

For a related reference, see Virtual Machine Definition.